Gyres of the world oceans http://staffwww.fullcoll.edu/tmorris/elements_of_ecology/images/ocean_currents.jpg

Gyres of the World Oceans: Wind-driven Ocean Circulation http://www.physicalgeography.net/fundamentals/8q_1.html

Wind-Driven Circulation: Motion of the Surface Ocean Ekman Transport - p. 138-139 Geostrophic Currents (gyres) - p. 140-141 Wind-Driven Upwelling - p. 142-143

p. 138 Ekman Spiral The prevailing winds create a drag (wind stress) on the ocean surface. The momentum gained at the surface is transferred deeper, but energy is lost with increasing depth. The Coriolis Effect deflects the moving water of the upper 75-150 m. A decrease in current speed coupled with continuous deflection with increasing depth creates a theoretical spiral of moving water called the Ekman Spiral.

Ekman Transport The surface current is ~45o to the prevailing wind. Adding all the vectors (magnitude and direction) of the Ekman Spiral yields a net current direction that is ~90o to the prevailing wind. This composite current is called Ekman Transport and it controls the motion of the surface ocean.

Ekman Spiral Ekman Transport Surface current what hemisphere does this example come from? Ekman Spiral change in direction and current velocity with increasing depth Northern Hemisphere - to the right of the prevailing wind Surface current (~45° to the prevailing wind) Ekman Transport = composite of Ekman Spiral; ~90o to the prevailing wind p. 138

Seeing the Ekman Spiral in 3-D: each layer of surface water moves in a different direction due to the Coriolis Effect

to the right of the prevailing winds in the N. Hemisphere, Ekman Transport ~90o to the prevailing winds 90° N (North Pole) 90° S (South Pole) Trade Winds (NE Trades) Polar Easterlies Westerlies to the right of the prevailing winds in the N. Hemisphere, to the left of the prevailing winds in the S. Hemisphere p. 133

Predict ocean circulation based on your knowledge of the prevailing winds and Ekman transport In the Northern Hemisphere: clockwise circulation in the low to mid-latitudes; counter-clockwise the high latitudes Circulation has the opposite sense in the S. Hemisphere

Wind-Driven Ocean Circulation Circulation of the upper water masses is set in motion by the energy of the prevailing winds Ekman transport is responsible for this wind-driven circulation Gyres are large horizontal current systems The subtropical gyres transport warm waters poleward along the western edges of the ocean basins, and cool waters equatorward along the eastern sides p. 132-133

http://www. miracosta. cc. ca http://www.miracosta.cc.ca.us/home/kmeldahl/currents/world_circulation.jpg

Geostrophic Currents Ekman transport and the Coriolis effect cause surface waters to converge (“pile-up”) in the subtropics, and diverge (move apart) at the equator and in subpolar waters. Convergence and divergence of surface water masses create subtle relief (“domes” and “depressions”) on the ocean surface (<2 m, or <6 ft.). Gravity acts on the water to pull it downslope, while the Coriolis effect works in the opposite direction. Geostrophic flow represents the partial balance between gravity and Coriolis. p. 134

p. 133 Ekman transport causes surface waters to converge (pile-up) in subtropical regions creating a hill or dome divergence convergence divergence convergence Ekman transport causes surface waters to diverge (move apart) in subpolar regions & at the equator creating a valley or depression divergence

Divergence creates valleys or depressions, Convergence produces hills or ridges.

divergence of water masses results in upwelling and high productivity in subpolar waters convergence of water masses causes near-surface waters to pile-up in the subtropics divergence of water masses results in upwelling and high productivity at the equator

Geostrophic currents represent the Profile Rotation of Earth Geostrophic currents represent the partial balance between gravity & Coriolis; most surface currents are geostrophic.

Western Boundary Currents Earth’s rotation from west to east, compounded by the typically strong Trade Winds, cause tropical & subtropical surface waters to “pile-up” on the western sides of the ocean basins The “hill” is displaced to the west of center causing water on the western side of the hill to flow faster than the eastern side In other words, the surface currents are forced through a narrower passage between the continents and the crest of the hill causing them to flow faster; this is called “western intensification”. Strong western boundary currents mark the western sides of the subtropical gyres. p. 134

Note: the dome is not in the center, but has been offset to the west p. 135 Map View Note: the dome is not in the center, but has been offset to the west

warm core ring cold core ring The Gulf Stream is a western boundary current of the subtropical gyre “Sargasso Sea”

http://veimages.gsfc.nasa.gov/1187/S1999148171230.png

The Gulf Stream and North Atlantic Drift warm Europe http://www.ospar.org/grfx/Q.F.2.4.jpg

Notice the warm currents flowing poleward on the western sides of the ocean basins and the cool currents flowing equatorward on the eastern sides. Also note the Antarctic Circumpolar Current that flows around the isolated continent of Antarctica; this current serves to connect ocean circulation between the three major ocean basins.

Lots of garbage collects here! The north Pacific Geostrophic Subtropical Gyre Nike shoes and toys found along North American beaches. Lots of garbage collects here! http://en.wikipedia.org/wiki/Image:North_Pacific_Gyre.png

Coastal Upwelling Map View Profile Surface waters are replaced by nutrient-rich deeper waters resulting in high biological productivity over the outer shelf and upper slope Ekman transport causes surface waters to be displaced seaward (i.e., pushed away from the coast)

Dark greens on land and light greens & reds in the oceans are areas of highest chlorophyll and plant/algae abundances. In the oceans this means that either upwelling or continental runoff has enriched the ocean in nutrients.

Oceanic Divergence (e.g., Equatorial Upwelling) Map View Profile Change in the direction of the Coriolis effect at the equator causes divergence of the surface waters Surface waters are replaced by nutrient-rich deeper waters resulting in high biological productivity

chlorophyll concentrations on land & sea Northern Hemisphere summer notice strong equatorial upwelling as well as coastal upwelling off Africa & Peru

1 2 3 1. equatorial upwelling (divergence) 2. coastal upwelling off Africa (several locations) 3. high latitude upwelling (divergence) around Antarctica 2 1 3

Summary of Wind-Driven Circulation p. 132-135 Summary of Wind-Driven Circulation divergence 1. Prevailing winds (Trades, Westerlies, Polar Easterlies) impart energy to set the upper water masses in motion (near-surface circulation) 2. Due to the Coriolis Effect, net transport of water is approximately at a right angle (~90o) to the direction of the prevailing winds (=Ekman Transport) 90o to the right in the N. Hemisphere 90o to the left in the S. Hemisphere 3. Ekman Transport causes near-surface waters to pile-up (converge) in subtropical regions (creating “hills”), and to move apart (diverge) in subpolar regions and at the equator (creating “valleys”) convergence divergence convergence divergence divergence of water masses causes upwelling of deeper waters

Lots of garbage collects here! Nike shoes and toys found along North American beaches. Lots of garbage collects here! http://en.wikipedia.org/wiki/Image:North_Pacific_Gyre.png

The “Great Pacific Garbage Patch” http://knowmanythings.files.wordpress.com/2009/07/great_pacific_garbage.jpg

The remnants of a Laysan Albatross chick which was fed plastic by its parents resulting in death. http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch

Geostrophic currents flow around subtle “hills” and “valleys” on the ocean surface

Summary of Wind-Driven Circulation 4. The major near-surface currents are geostrophic, that is, the water masses flow around subtle “hills” and “valleys” on the ocean surface (balance between Coriolis effect & gravity) e.g., the subtropical gyres flow around “hills” of the Atlantic, Pacific & Indian oceans 5. Near-surface currents are stronger on the western sides of the subtropical gyres (=western boundary currents) the water is piled-up towards the west because of the Earth’s rotation and the strong Trade winds; currents flow faster where the “hills” are steepest & narrowest (i.e., the west side of the hill) 6. Western boundary currents transport warm waters poleward; eastern boundary currents transport cool waters equatorward. p. 132-135